Enhanced Rice Growth and Whole Plant Metabolism Is Conferred by Increased Leaf and Seed ADP-Glucose Pyrophosphorylase Activity

Leaf and seed starch biosynthesis are both important to cereal crop yield in that carbohydrates are stored transiently as starch in leaves and long term as starch in seeds.   ADPglucose pyrophosphorylase (AGP) is the rate-limiting step in starch biosynthesis.  The lack of AGP, and starch production, in leaves or seeds diminishes plant productivity while AGP up-regulation has been shown to increased yield.  In our experiments we are testing the role of rice leaf and seed AGP upon whole plant productivity and global gene expression.   Data obtained from replicated yield trials indicates that overexpression of both leaf and seed AGP is beneficial to plant growth and produces greater yield increases than observed with expression of the leaf or seed AGP transgene alone.  In fact, plants overexpressing the seed transgene did not exhibit yield increases, but those overexpressing the leaf transgene had significantly increased biomass (24%) and seed yield trended 8% higher in the form of increased seed number.  Plants with expression of both the leaf and seed starch transgenes had significantly increased biomass (60%) over the wild type and seed yield trended 16% higher.   Global expression analysis and measurements of leaf metabolites indicates that overexpression of leaf and seed AGP leads to increased expression of several starch biosynthetic genes and increased levels of several key metabolites.